Ultra high strength ductile iron

ABSTRACT

DIRECTED TO ALLOYED DUCTILE IRONS HAVING IN THE ASTEMPERED CONDITION A YIELD STRENGTH (0.2% OFFSET) OF AT LEAST ABOUT 150,000 P.S.I., AND EVEN AT LEAST ABOUT 170,000 P.S.I., WHICH CONTAIN ABOUT 2.6% TO ABOUT 4.0% CARBON, ABOUT 1.5% TO 4% SILICON, ABOUT 6% TO 11% NICKEL, UP TO ABOUT 7% COBALT, AND EFFECTIVE AMOUNT OF A GRAPHITE SPHERIODIZING AGENT, UP TO ABOUT 1% MANGENESE, UP TO ABOUT 0.4% MOLYBDENUM AND THE BALANCE ESSENTIALLY IRON.

NOV. 7; 1972 c -lu c 3,702,269

ULTRA HIGH STRENGTH DUCTILE IRON Filed Jan. 22, 1971 .INVENTOR. NR THHNL. CHURCH United States Patent Ofice Patented Nov. 7, 1972 3,702,269ULTRA HIGH STRENGTH DUCTILE IRON Nathan Lewis Church, Warwick, N.Y.,assignor to The International Nickel Company, Inc., New York, N.Y.Continuation-impart of abandoned application Ser. No.

878,938, Nov. 21, 1969. This application Jan. 22, 1971,

Ser. No. 108,702

Int. Cl. C22c 37/00, 37/04 U.S. Cl. 148-35 4 Claims ABSTRACT OF THEDISCLOSURE Directed to alloyed ductile irons having in the astemperedcondition a yield strength (0.2% ofiset) of at least about 150,000p.s.i., and even at least about 170,000 p.s.i., which contain about 2.6%to about 4.0% carbon, about 1.5% to 4% silicon, about 6% to 11% nickel,up to about 7% cobalt, an effective amount of a graphite spheroidizingagent, up to about 1% manganese, up to about 0.4% molybdenum and thebalance essentially lron.

The present application is a continuation-in-part of U.S. applicationSer. No. 878,938, filed Nov. 21, 1969, now abandoned.

Industry is demanding in many areas metallic materials having higher andhigher strength together with freedom from production difficulties. Thisis particularly true with respect to the demands being placed onfoundries which produce steel and cast iron castings. It is possible toprovide very high strengths in many castings made of alloy steels.However, in order to produce high strengths, e.g., yield strengths (0.2%offset) on the order of at least 150,000 pounds per square inch to180,000 pounds per square inch in steel castings a relatively complexand expensive heat treatment is required. Such heat treatments usuallyinvolve a quench from an elevated temperature followed by a temperingtreatment. The quenching treatment causes severe thermal stresses insteel castings With the result that a substantial number of apparentlysatisfactory castings are scrapped due to quench cracking and distortionencountered during heat treatment. The resulting scrap losses togetherwith the high cost due to heat treatment has limited the application ofsteel castings in many areas, particularly those in which castings ofcomplex section are involved.

It would be desirable to employ castings made of ductile iron instead ofsteel castings, if the requisite strength could be developed in ductileiron. Thus, the castability of alloyed ductile iron is substantiallybetter than that of alloy steels and this is of advantage in producingcastings of complex section. Presently available alloyed ductile ironcastings are capable of providing yield strengths of up to about 100,000pounds per square inch and it would be very desirable to provide ductileiron castings capable of developing a markedly higher strength level.

I have now discovered alloy ductile iron castings capable of providingyield strengths after a simple tempering treatment on the order of atleast about 150,000 pounds per square inch to about 180,000 pounds persquare inch.

It is an object of the present invention to provide alloyed ductile ironcastings having a high yield strength on the order of at least about150,000 pounds per square inch (p.s.i.) after a simple temperingtreatment.

Other objects and advantages of the invention will become apparent fromthe following description taken in conjunction with the drawing which isa reproduction of a photomicrograph taken at 250 diameters depicting thefine lower bainite microstructure obtained in spheroidal graphitecastings of the invention.

Broadly stated, the present invention is directed to alloyed ductileiron compositions containing about 2.6% to about 4.0% carbon, about 1.5%to about 4% silicon, at least about 6% up to about 11% nickel, up toabout 7% cobalt, up to about 1% manganese, up to about 0.3%, or even0.4%, molybdenum, a small amount up to about 0.1% magnesium eliective tocontrol graphite in the cast iron to the spheroidal form and the balanceessentially iron. Castings produced within the aforedescribedcompositional range develop high yield strengths, e.g., 150,000 poundsper square inch, after a simple tempering treatment in the range ofabout 400 F. to about 700 F. e.g., about 500 F. to about 600 F.,conducted for times of at least about 2 hours up to about 8 hours, andare characterized by a fine lower bainite microstructure.

Preferred alloys within the invention, which provide exceptionally highyield strengths in the as-ternpered condition in section sizes up tofour inches and even greater contain about 2.9% to 3.6% carbon, about0.1% to 0.4% manganese, about 2% to about 2.5% silicon, about 7% to 9%nickel, about 0.1% to about 0.3% molybdenum, about 0.04% to about 0.07%magnesium and the balance essentially iron. Such alloys arecharacterized by spheroidal graphite, a fine matrix structure of lowerbainite, a high yield strength usually exceeding about kilo pounds persquare inch (k.s.i.) with the accompanying useful ductility and impactvalues.

A compositional range for other alloys within the invention which willgenerally provide irons having yield strengths of at least about 160,000p.s.i., in the as-tempered condition comprises about 3% to about 3.6%carbon, about 0.15% to about 0.25% manganese, about 2.2% to about 2.5silicon, about 8% to about 10% nickel, about 2% to about 4% cobalt, upto about 0.24% molybdenum, about 0.045% to about 0.06% magnesium and thebalance essentially iron. Such castings in the as-tempered condition arecharacterized by a microstructure comprising essentially lower bainite,with the graphite being present in the spheroidal form.

The most significant alloy constituent in the irons of the invention isnickel. Thus, nickel within the ranges employed, promotes the formationof fine lower bainite in the microstructure accompanied by a yieldstrength of at least about 150,000 p.s.i. and toughens the matrix. Ifnickel is reduced below about 6%, undesirable 'weaker upper bainite orpearlite will form thereby decreasing the attainable strength, whereasnickel contents exceeding about 10% or 11% increase the amount ofmartensite in the structure thereby limiting ductility. In addition, toohigh nickel contents can result in the presence of retained austenite inthe structure thereby decreasing strength. Cobalt, when used inconjunction with nickel, acts to limit retained austenite formation. Ifcobalt is too high, the strength will be reduced. As the casting sectionsize increases above about one inch, more nickel is employed to preservethe fine lower bainite microstructure and to overcome the tendency forthe coarser upper bainite structure to form, a result which wouldmarkedly reduce yield strength.

Carbon is essential in the iron to provide graphite. Carbon contentsbelow 2.4% yield brittle semi-carbidic irons with an undesirableintercellular carbide structure with reduced castability. On the otherhand, carbon contents exceeding about 3.6% result in lowered strength.Silicon is also an important constituent of the iron, since it promotesgraphitization of the alloy and helps to decrease eutectoid carboncontent. Hence, the silicon content is at least about 1.5% to aid inpreventing carbide formation, but does not exceed about 4% as otherwisethe matrix tends to become embrittled. Increasing silicon raises impacttransition temperatures. Advantageously,

silicon is about 1.9% to about 2.5% to provide high yield strengths inconjunction with nickel and the other constituents of the alloy.Magnesium is present in the iron for the purpose of spheroidizinggraphite. Other known graphite spheroidizers such as cerium, lanthanumand other rare earth metals of the Lanthanide Series may be employed bythemselves or in conjunction with magnesium in amounts up to about 0.1%for this purpose. Manganese is not required in the alloys, but may beemployed in amounts up to 1% in the higher strength irons to promote theformation of lower bainite. Manganese promotes an undesirable stableaustenite and can cause carbide problems in heavier sections whenpresent in excessive amounts. Molybdenum may also be employed in amountsup to about 0.3%, or, in some cases, even up to 0.4%, e.g., about 0.2%,as a strengthening element in conjunction with nickel, particularly inthe special cobaltfree irons and in heavier-section castings to preservehigh strength. Chromium is not employed in the alloys since thispowerful carbide former exerts an undesirable embrittling effect whichbecomes particularly aggravated in heavier sections and which in heaviersections may be accompanied by segregation. Accordingly, the chromiumcontent does not exceed 0.2%.

The ability of the irons to develop high strength after a simpletempering treatment is an outstanding advantage. Thus, the temperingtreatment may readily be performed in equipment available at mostfoundries. Since no quenching is required, the possibility of quenchcracking and distortion is eliminated. Particularly with reference tothe higher strength alloys, as described hereinbefore, it is found thatessentially no dimensional changes occur during tempering. This is animportant advantage particularly in applications requiring castings ofcomplex configuration. It has been found that heat treatments conductedat austenitizing temperatures, e.g., 1450 F., followed by isothermaltransformation at temperatures on the order of 450 F. to 500 F. followedby tempering, gave poorer combinations of strength and ductility thanwere obtained by simply tempering mold cooled castings.

In order to give those skilled in the art a better understanding of theinvention a series of heats was prepared by induction melting charges ofhigh grade pig iron, electrolytic nickel, electrolytic cobalt (whencobalt was employed) and standard ferroalloys. In instances whereferromanganese was added, it was added after melt-down at a bathtemperature of 2750 F. and the bath was then heated to 2850 F., heldfive minutes and cooled to 2750 F. In each instance, after melt-down theiron was treated at a temperature of about 2750 F. with a nickelmagnesium alloy to introduce magnesium and was then given a graphitizinginoculation comprising an addition of 0.5% silicon as a calcium-bearingferrosilicon alloy containing 85% silicon and metal from the treatedmelt was then cast into sand molds. The castings produced were keelblocks. The compositions of alloys produced within the invention are setforth in the following Tables I and V for nickel and for nickel-cobaltirons, respec tively, and the results of tensile testing performed uponthe tempered castings are set forth in the following Tables II, III, IVand VI. The casting section size and tempering heat treatment conditionsare given in each of Tables II, III, and IV, and the casting sectionsize was one inch for the results reported in Table VI.

TABLE II 1" Section (4 hr.600 FJAC) YS at CVN at 0.2 Percent roomHardofiset U TS el. in R.A., temp. ness (K s.i.) (K SJ.) 1 percent(it-lbs.) (R5) 150.3 105. 3 4 0 5.0 44.4 152. 4 102. 3 3 5 5. 0 44. 0 1170.4 210.4 2 3 4.0 40.0 I 177.3 210.5 2 2 4.0 40.7 1 100.3 203.4 2 34.0 40.2 1 105.7 100.0 1 2 4.0 40.4 1 175.2 215.7 3 3. 5 4.5 47.2 1174.7 200.4 2 3.5 4.0 40.0 5 172.0 213.0 2 4.5 4.0 47.0 170.0 200.2 23.5 4.0 45.4 0 105.3 171.3 1 4 22:; 7 1 176.8 207.5 1 4 4.0 47.7 "1175.1 202.2 1 Nil 4.0 40.0

TABLE III 2" Section (4 hr./600 FJAC) YS at CVN at 0.2% Percent roomHard- I ofiset U'IS el. in R.A., temp. 11855 Alloy No. (K s.i.) (K 5.1.)1 percent (it-lbs.) (R

5 130.3 103.7 5 0 1 "1 122.3 ig 0 3.2 i

p103 102.1 3 i M 107.7 202.; i i 140. 0 17 6 "1 140. 7 .2 12.2 i

2" Section (4 hr./500 FJAC) YS at CVN at 0.2% Percent room Hardofiset UTe1. in R.A., temp. ness (K s.i.) (K s.i 1 percent (ft-lbs.) (R

i g 1 2 7.0 37.1 00.0 151.0 100.2 2; 1 g i 100.7 104. 12 -7 200 a 0 i 0.157. z; t 105. 7. 102.4 200.4 2 i M 140.2 100.7 140.2 ns 1.2 i 157.0 0.0 1. 150.0 102.2 1 1 i l Broke in specimen neck.

TABLE IV 4 Section (4 Ina/600" FJAC) Ys at CVN at:

0.2% Percent room Hardofis'et U S 01. in R.A., temp. ness Alloy No. (K5.1.) (K 5.1 1" percent (it.-lbs.) (R.,)

ps2 2 101; E 5

gag 3313 i i 5 103.0 182.5 1 Nil 1 Broke in specimen neck.

The data of Tables II through IV demonstrate that the higher nickelirons retain high strength (and the desired lower bainitemicrostructure) as section size is increased above one inch. Alloy 6serves to illustrate the desirability of maintaining silicon above 1.9%to provide high yield strength, although, as shown in Table III, Alloy 6is indeed strong.

The photomicrograph depicted in the drawing was derived from the castingof Alloy No. 3, the properties for which are given in Table II. The veryfine lower bainite microstructure is clearly evident therefrom.

TABLE V Composition (wt. percent) Mn Si Ni M0 Mg TABLE VI.MECHANICALPROPERTIES Heat Y.S. (p.s.i.) Hardtreat- 0.2% U.T.S. El., R.A., uess,Alloy No. ment 1 ofiset (p.s.i.) percent percent B11 Heat treatmentcode: A=4 hrs/500 FJAC; B=4 hrs/500 FJAO plus 4 hrs/600 FJAC; C=4hrs/600 FJAO.

Standard Charpy V-Notch impact specimens were prepared from Alloys 22,23 and 27 and were tested at room temperature with impact values in therange of 3 to 3.5 foot-pounds being determined.

While the data in Table VI was obtained upon one inch castings, theresults of other tests have indicated that strengths of a high order,together with substantial retention of hardness and impact values, areobtained in heavier castings, e.g., castings having sections up to fourinches thick. The fact that Charpy V-notch values were obtained in ironsat the high strength levels demonstrated hereinbefore illustrates theremarkable combinations of properties possessed by alloys within theinvention. It is to be remembered in this connection that conventionalpearlitic ductile irons provide essentially zero Charpy V-notch valuesat strengths on the order of 80,000 p.s.i.

Alloys provided in accordance with the invention are machinable andcastings made therefrom can be employed in the production of complexparts such as high pressure centrifugal gas compressor impellers, pumpparts, etc.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. A ductile iron sand casting characterized in the tempered conditionby a fine lower bainite structure consisting essentially of 2.6% toabout 4.0% carbon, about 1.5% to about 4% silicon, about 6% to about 11%nickel, up to about 7% cobalt, up to about 1% manganese, up to about0.4% molybdenum, not more than about 0.2% chromium, a small amount up toabout 0.1% of a graphite spheroidizing agent effective to promote theoccurrence of spheroidal graphite in said ductile iron and the balanceessentially ll'OIl.

2. A ductile iron casting in accordance with claim 1 containing about2.9% to about 3.6% carbon, about 0.1% to 0.4% manganese, about 2% toabout 2.5% silicon, about 7% to about 9% nickel, about 0.1% to about0.3% molybdenum, and about 0.04% to about 0.07% magnesium.

3. A bainitic ductile iron casting in accordance with claim 1 whereinthe graphite spheroidizing agent is selected from the group consistingof magnesium and a metal of the Lanthanide Series.

4. A bainitic ductile iron casting in accordance with claim 1 containingabout 3% to about 3.4% carbon, about 2.2% to about 2.5% silicon, about8% to about 10% nickel, about 2% to about 4% cobalt, about 0.15% toabout 0.25% manganese, up to about 0.24% molybdenum, and about 0.045% toabout 0.06% magnesium and having a matrix microstructure comprisingessentially lower bainite.

References Cited UNITED STATES PATENTS 1,496,979 6/1924 Corning 148-352,046,913 7/1936 Kormann l23 CB 3,273,998 9/1966 Knoth 148-35 X3,549,430 12/1970 Kies 148-35 2,485,760 10/1949 Millis et al. 75l23 CB2,516,524 7/1950 Millis et al 75l23 CB 2,970,902 2/1961 Alexander et al.75-123 CB 3,125,442 3/1964 Alexander 75l23 CB L. DEWAYNE RUTLEDGE,Primary Examiner I. E. LEGRU, Assistant Examiner US. Cl. X.R. 75--123CB, 128 C

